Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of iron, nickel and cobalt-base superalloys. However, components formed from superalloys must often be provided with some form of thermal and/or environmental protection in order to exhibit adequate service lives in certain sections of a gas turbine engine, such as the turbine, combustor and augmentor.
Silicon carbide (SiC)-based ceramic matrix composite (CMC) materials have been proposed as materials for certain components of gas turbine engines, such as the turbine blades and vanes. Various methods are known for fabricating SiC-based CMC components, including melt infiltration (MI), chemical vapor infiltration (CVI) and polymer pyrolysis (PIP) processes. Though these fabrication techniques significantly differ from each other, each involves the use of tooling or dies to produce a near-net-shape part through a process that includes the application of heat at various processing stages. As with turbine blades and vanes formed of more conventional superalloy materials, CMC blades and vanes are preferably equipped with cooling passages and holes in order to reduce their operating temperatures. Cooling passages and holes, as well as other cavities, are typically formed in CMC components using a combination of removable and expendable tooling. The external contours of hollow CMC components are typically formed using removable tooling that can be reused in most cases. Internal cavities can also be formed using removable tooling, though conventional silica (SiO.sub.2) and alumina (Al.sub.2 O.sub.3) cores widely used with investment casting methods have also been used.
Silica and alumina cores require removal with a leaching compound, including salts, hydrogen fluoride (HF) and alkalis such as sodium hydroxide (NaOH) and potassium hydroxide (KOH). In some cases, the exposed surfaces of a metal investment casting are coated with a masking material to prevent surface attack by the leaching compound--the internal surfaces of the casting cannot be masked due to the presence of the core. As a result, the critical external surfaces of the casting are protected, while less critical internal surfaces are subject to mild attack by the leaching compound. However, leaching compounds conventionally used to remove silica cores from investment castings aggressively attack many CMC materials, and particularly those (that contain silicon and boron, typically in the form of SiC and boron nitride (BN), respectively. Accordingly, attempts to remove silica cores from CMC components susceptible to attack by leaching compounds suffer unacceptable attack of its internal surfaces, which reduces the structural integrity of the CMC component.
Accordingly, it would be desirable if a method were available for fabricating a CMC component with an internal cavity formed by a core that can be readily removed without damaging the walls of the cavity.